Engine knock and oil pressure gauge problem

My 1995 Chevy Silverado (5.7L) is making a knocking when i first start it up (lasting a few seconds at most) at the same time my oil pressure gauge sits at zero until the knock stops the goes back to normal. when the truck is warm and i am driving the oil pressure gauge will drop to zero, it returns back to normal once i am moving. I am not having any issues with performance.

could my oil pump need to be replaced? or would this be something else.